1) Field of the Invention
The present invention relates to a toner for MICR (magnetic ink character recognition) capable of printing magnetic images by a printer of one-component magnetic developing system.
2) Description of the Related Art
In recent years, documents capable of magnetic ink character recognition (MICR) and particularly checks or bills have been easily prepared by one-component magnetic developing system with a magnetic toner. The MICR that is a system of reading magnetized images by a magnetic head is not conveniently used because images are obtained by offset printing with magnetic ink. A process of printing with a two-component developer which has been put to practical use is not also conveniently used, because the process requires a large-sized machine as compared with that for the one-component developer. As small-sized printers, there are those for heat-sensitive transfer processes. However, almost all of them are single-purpose machines for printing only MICR characters. It is accordingly desired to develop a small-sized printer capable of printing characters or symbols together with the MICR characters. Regarding the use for MICR, the one-component developing process has been developed hitherto because of using a compact machine, keeping easy maintenance and being capable of printing images other than MICR characters.
In the prior art, magnetic materials having large magnetization were attempted to use for the toner for MICR. Japanese Laid-open Patent Publication Nos. Hei 6-282100 and Hei 7-271085 discloses the use of acicular magnetite. The acicular magnetite has, however, problems that it is easily exposed on the surface of toner particles and is easily released from the toner particles by sliding friction with a magnetic head. Although appropriate saturation magnetization is necessary for development by the one-component magnetic developing process, signal strength becomes too high when the acicular magnetite is added in an amount necessary to development. The amount of the acicular magnetite is restricted because of its inferior dispersing ability. It is therefore difficult to satisfy both of the saturation magnetization required for development and the residual magnetization required for signal strength. It is furthermore impossible to satisfy every requirement even if it is used together with magnetite of other type. Moreover, the toner containing magnetite generates lacking or omission of characters in magnetic image formed by the development and, consequently, troubles are often caused upon magnetic reading.
The signal strength is influenced by a deposition amount of the toner, and the deposition amount of the toner is influenced by charging property of the toner. It is therefore very important for the toner for MICR to maintain the stabilized charging property. In order to control the charging property, charge controlling agents are generally used. Selection of the charge controlling agent, however, is not easy to carry out, because the charging property is also influenced by the magnetic material.
The Japanese Laid-open Patent Publication Nos. Hei 6-282100, 6-43689 and 7-271085 discloses addition of various kinds of waxes to a toner for MICR in order to improve resistance against sliding friction. The resultant toner however often causes problems in magnetic reading even if the image formed has no problem in reading by eyes.
In the prior arts, as above-mentioned, there is no MICR toner which satisfies excellent resistance against sliding friction with the magnetic head and appropriate signal strength and forms magnetic images having stabilized image density and good image quality upon copying a number of sheets, without hurting image density and image quality such as, fog, lacking or omission of characters, fine line reproducibility, etc.
An object of the present invention is accordingly to provide a toner for MICR having sufficient resistance against sliding friction with the magnetic head, having appropriate signal strength by which reading errors are not caused, and having no trouble in image qualities such as image density, fog, etc.
Another object of the present invention is to provide a toner for MICR having stabilized charging property, which keeps appropriate toner images capable of reading by the magnetic head.
A further object of the present invention is to provide a MICR toner for MICR capable of forming magnetic images having stabilized image quality upon copying a large number of sheets. A furthermore object of the present invention is to provide a toner having good transferability and excellent fine line reproducibility and forming images without causing lacking or omission of characters.
A toner for MICR of the present invention comprises a binder resin, magnetite particles comprising a mixture of granular magnetite and acicular magnetite, and a wax, wherein a ratio by weight of said acicular magnetite in said magnetite particles is 0.1-0.5 to the granular magnetite of 1.0, and said magnetite particles are included in an amount of 15-50% by weight in the toner.
In the MICR toner according to the present invention, said granular magnetite is preferred to have residual magnetization of 5-15 emu/g and saturation magnetization of 70-95 emu/g, and said acicular magnetite is preferred to have residual magnetization of 20-50 emu/g and saturation magnetization of 70-95 emu/g.
In the present invention, the above-mentioned wax is preferred to have a melting point measured by DSC (referred as to xe2x80x9cDSC melting pointxe2x80x9d hereinafter) of 60-100xc2x0 C., and a preferable wax is natural gas-type Fischer-Tropsch wax. It is also preferred that the toner contains a charge controlling argent which preferably consists of at least two charge controlling materials, at least one of which is a chrome azo dye.
The MICR toner according to the present invention may contain a silicone oil and an inorganic fine powder on the surface of the toner particles. In such a case, it is preferred that the silicone oil has the viscosity of 10-1000 centistokes at 25xc2x0 C. and contains the volatile ingredients of 1.5% by weight or less and that the amount of the silicone oil is in a range of 0.01-0.5% by weight.